Summary: Most work on the relationship between farm size and productivity strongly suggests that farms that rely mostly on family labor are more productive than large farms operated primarily by hired labor. This study began as an inquiry into how rental and sales markets for agricultural land in the developing world affect efficiency and equity. What emerged was the clear sense that great variations in land relations around the world and over time cannot be understood in the common paradigm of property rights and competitive markets. Under that paradigm, land scarcity leads to better definition of rights, which are then traded in sales and rental markets accessible equally to all players. The outcome should be the allocation of land to the most efficient uses and users, yet this rarely happens. Instead, land rights and ownership tend to grow out of power relationships. Landowning groups have used coercion and distortions in land, labor, credit, and commodity markets to extract economic rents from the land, from peasants and workers, and most recently from urban consumer groups or taxpayers. Such rent-seeking activities reduce the efficiency of resource use, retard growth, and increase the poverty of the rural population. The authors examine how these power relations emerged and what legal means enabled relatively few landowners to accumulate and hold on to large landholdings. The authors discuss the successes and failures of reform in market and socialist economies, and the perversions of reforms in both systems, manifested in large commercial farms and collectives. They survey the history of land relations and the legacies that history leaves. They discuss the three analytical controversies surrounding economies of scale, and the efficiency of the land sales and land rental market. They discuss the main policy issues and implications of various distortions and successful and unsuccessful reforms in the developing world, including land registration and titling, land taxation, regulations restricting land sales and rentals, fragmentation and consolidation of land, redistributive land reform, and decollectivization. In an epilogue on methodology, the authors examine how various strands of economic theory have contributed, or failed to contribute, to the explanation of variations in policies, distortions, and land relations over space and time.